System and apparatus for determining ambient temperatures for a fluid analyte system

ABSTRACT

A system and method for rapidly determining ambient temperature in a fluid-analyte meter. The meter includes a housing defining an interior space and an area for receiving a fluid sample. A processor and a first temperature sensor are disposed within the interior space of said the housing. A second temperature sensor is disposed on the housing. One or more processors are configured to determine a first temperature value from temperature data received from the first temperature sensor. The processor(s) are also configured to apply a variable current to a temperature-adjustment source such that the second temperature sensor is adjusted to a predetermined steady-state temperature value different from the first temperature value. The processor(s) are further configured to determine an ambient temperature of an exterior space of the housing based on the applied variable current, pre-determined steady-state temperature, and received first temperature values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 61/385,749, filed Sep. 23, 2010, which is herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to systems for determiningambient temperature, and more particularly, to the determination ofambient temperature for a device in which the temperature inside thedevice is different from the ambient temperature.

BACKGROUND

The quantitative determination of analytes in body fluids is of greatimportance in the diagnoses and maintenance of certain physiologicalconditions. For example, lactate, cholesterol, and bilirubin should bemonitored in certain individuals. In particular, determining glucose inbody fluids is important to individuals with diabetes who mustfrequently check the glucose level in their blood to regulate thecarbohydrate intake in their diets. The results of such tests can beused to determine what, if any, insulin or other medication needs to beadministered. In one type of testing system, test sensors are used totest a fluid such as a sample of blood.

Measurement of blood glucose concentration is typically based on achemical reaction between blood glucose and a reagent. The chemicalreaction and the resulting blood glucose reading as determined by ablood glucose meter is temperature sensitive. Therefore, a temperaturesensor is typically placed inside the blood glucose meter. The ambienttemperature and reagent temperature are then extracted using thetemperature sensor readings. The calculation for blood glucoseconcentration in such meters typically assumes that the temperature ofthe reagent is the same as the temperature reading from the test sensorplaced inside the meter. However, if the actual temperature of thereagent and the test sensor are different, the calculated blood glucoseconcentration will not be as accurate. An increase in temperature or thepresence of a heat source within a blood glucose meter will generallyresult in erroneous blood glucose measurements. Furthermore, the thermalproperties of a blood glucose meter often render the system slow torespond to environmental changes such as a change in temperature.

SUMMARY

According to one embodiment, a fluid-analyte meter is configured torapidly determine ambient temperature. The meter includes a housingdefining an interior space and an exterior ambient space. A processor isdisposed within the housing. A controller is disposed within thehousing. The controller is communicatively connected to the processor. Afirst temperature sensor is disposed within the interior space of thehousing. The second temperature sensor is disposed on the housing. Thesecond temperature sensor is communicatively connected to thecontroller. A temperature-adjustment source is disposed on the housing.The temperature-adjustment source is configured to create a convectivezone about the second temperature sensor. The controller is configuredto transmit instructions for supplying a variable current to thetemperature-adjustment source such that the temperature-adjustmentsource adjusts the second temperature sensor to a predeterminedsteady-state temperature value different from a first temperature value.The first temperature value is based on first temperature data receivedvia the controller from the first temperature sensor. The controller isfurther configured to receive second temperature data from the secondtemperature sensor. The controller or the processor are configured todetermine an ambient temperature of the exterior ambient space based onthe supplied variable current, predetermined steady-state temperature,and first temperature values.

According to another embodiment, a method for rapidly determiningambient temperature in a fluid-analyte meter includes providing a meterincluding a housing. A first temperature sensor is disposed within aninterior space of the housing. A second temperature sensor is disposedon the housing. A temperature-adjustment source is disposed near thesecond temperature sensor. A first temperature value is determined fromfirst temperature data received via the first temperature sensor. Avariable current is applied to the temperature-adjustment source suchthat the second temperature sensor is adjusted to a predeterminedequilibrium temperature value different from the first temperaturevalue. A second temperature value is determined from second temperaturedata received from the second temperature sensor. An ambient temperatureof an exterior space of said fluid-analyte meter is determined, via oneor more processors, based on the variable current and the determinedfirst and second temperature values.

According to another embodiment, a portable meter configured to rapidlydetermine ambient temperature includes a housing defining an interiorspace and an area for receiving a fluid sample. A first processing unitand a first temperature sensor are disposed within the interior space ofthe housing. A second temperature sensor is disposed on the housing. Thefirst processing unit or another processing unit is configured todetermine a first temperature value from temperature data received fromthe first temperature sensor. A variable current is applied to atemperature-adjustment source such that the second temperature sensor isadjusted to a predetermined steady-state temperature value differentfrom the first temperature value. An ambient temperature of an exteriorspace of the housing is determined based on the applied variablecurrent, the pre-determined steady-state temperature value, and thereceived first temperature value. The ambient temperature value isdetermined in less than about sixty seconds.

Additional aspects of the invention will be apparent to those ofordinary skill in the art in view of the detailed description of variousembodiments, which is made with reference to the drawings, a briefdescription of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a temperature sensor disposed on a casing of a meteraccording to one embodiment.

FIG. 2 illustrates a temperature sensor disposed on a casing of a meteraccording to another embodiment.

FIG. 3 illustrates a fluid analyte system including a meter and a testsensor according to one embodiment.

FIG. 4 illustrates a front view of a portable device according toanother embodiment.

FIG. 5A illustrates a portable fluid analyte device with a USB interfaceaccording to another embodiment.

FIG. 5B illustrates a side view of the portable device of FIG. 5A.

While the invention is susceptible to various modifications andalternative forms, specific embodiments are shown by way of example inthe drawings and are described in detail herein. It should beunderstood, however, that the invention is not intended to be limited tothe particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention.

DETAILED DESCRIPTION

Generally, a test sensor is employed to collect a fluid sample, such asblood, and a blood glucose meter measures a reaction between the glucosein the fluid sample with a reagent on the test sensor to calculate acorresponding blood glucose concentration. The temperature of thereagent affects the reaction between the glucose and the reagent. Assuch, the temperature of the reagent also affects the blood glucoseconcentration calculated by the blood glucose meter. For certainembodiments, such as, where the reagent area of the test sensor islocated outside the blood glucose meter, the temperature of the reagentcan be assumed to be substantially equal to the ambient temperature inor surrounding the meter. Temperature sensing elements in the bloodglucose meter can provide an estimate of the ambient temperature, whichcan then be used in the calculation of the blood glucose concentration.However, the blood glucose meter includes various heat-generatingelements, which along with various external heat sources, can cause thetemperature measured by the temperature sensing elements in the meter todiffer from the ambient temperature. When the temperature measured bythe temperature sensing elements does not provide an accurate estimateof the ambient temperature, inaccuracies are introduced into thedetermination of the blood glucose concentration.

To achieve more accurate determinations of fluid-analyte concentrations(e.g., blood glucose concentration), different temperature sensorconfigurations can be used to determine ambient temperature for aninstrument (e.g., blood glucose meter, other fluid-analyte meter). Inone embodiment, a resistance temperature detector (RTD) sensor can beembedded in, or otherwise disposed on the interior or exterior surfaceof an instrument casing or shell. This can be beneficial for determiningambient temperature by providing temperature-related data, which canthen be adjusted to compensate for any of a number of factors, such asreagent errors caused by ambient temperature variations, temperaturevariations caused by internal heating of the instrument by internalelectronic components, or nearby localized heating from externalsources.

In certain embodiments, test sensors (e.g., test strips) are used inconjunction with a fluid-analyte meter. In such embodiments, a teststrip may include electrical components for interfacing the test stripwith a fluid-analyte meter. Test strips tend to quickly equilibrate tothe ambient temperature of the testing environment, such as areas withinor surrounding a fluid-analyte meter. To accurately determine analyteconcentrations for test samples received via a test strip, it isbeneficial to correctly estimate the ambient temperature particularlywhere direct temperature measurement on a test strip is not possible. Itis also desirable for the ambient temperature estimate to be madequickly and accurately, particularly for meters exposed to testingenvironments that change often.

Exemplary and non-limiting temperature sensor embodiments fordetermining the ambient temperature at or surrounding an instrument,such as a fluid analyte meter, are illustrated in FIGS. 1 and 2.Non-limiting systems that may employ these temperature sensorembodiments are described below in connection with FIGS. 3-5.

Referring now to FIGS. 1 and 2, non-limiting and exemplary temperaturesensor configurations are illustrated for determining ambienttemperature using an RTD sensor embedded in or otherwise disposed on theinterior or exterior of a fluid-analyte meter. One or more RTD sensorsmay be disposed on, or at least partially embedded in, the shell orcasing of the fluid analyte meter. The RTD sensor embodiments describedherein provide a relatively quick and accurate estimation of ambienttemperature, despite, for example, the ambient temperature beingunknown. These RTD sensor configurations are at least partially based onconvection around the meter due to temperature differences between themeter and its surrounding environment and increasing the temperature ofthe RTD sensor.

In certain embodiments, either the RTD sensor, or a separate heat sourcedisposed near or adjacent to the RTD sensor, can be configured togenerate heat flow into (e.g., from the ambient environment to theinside the meter) and out of the meter (e.g., from inside the meter tothe ambient environment). For example, a current may be applied to theRTD sensor or the heat source to heat the RTD sensor to a temperaturethat is higher than the internal temperature of the meter. The benefitof heating the RTD sensor to a temperature above the internaltemperature of the meter is to create air flow about the meter, ratherthan the RTD sensor measuring a surface temperature based on stagnantair immediately adjacent the meter casing. Through localized heating ofthe RTD sensor, localized convection is promoted around the meter suchthat the heat flow or heat loss from the RTD sensor allows an accuratedetermination of ambient temperature for use in any subsequentdeterminations of fluid-analyte concentration based on the determinedambient temperature.

The internal temperature of a meter may be determined by the RTD sensoritself, or desirably by a separate temperature sensor (e.g., anotherRTD, a thermistor, a thermocouple, semiconductor diodes) locatedinternal to or within the meter. In certain embodiments, a very lowcurrent may be supplied to a separate temperature sensor that is used todetermine the internal temperature of a meter such that the separatetemperature sensor does not raise the temperature of the meter. When theRTD sensor is used to determine the internal temperature of the meter, asimilar procedure is used where a very low current is applied to the RTDsensor before a higher current is applied for heating purposes. Also,similarly, the very low current that is applied is such that the RTDsensor during the temperature determination stage contributes anegligible temperature rise to the meter. As discussed previously, theinternal temperature of a meter can increase due to external factorsalong with heat generated by certain electronic components of the meter,such as a display, a microprocessor, or other electronic components.

The thermal mass of a meter (see, e.g., meters 100, 200, 300), such as,for example, a fluid-analyte meter, is typically much larger than thethermal mass of an RTD sensor that is used to determine ambienttemperature. The RTD sensor or heat source will therefore generallyprovide localized heating in a small region of the meter in the vicinityof the heat source. Thus, the localized heating is not expected tosignificantly affect the internal temperature of the meter. However, itcan be desirable to locate the RTD sensor or similar heat sources awayfrom elements on the meter that are heat-sensitive.

In certain embodiments, an RTD sensor is desirably heated so that theRTD sensor maintains a constant or almost constant temperature above theinternal temperature of the meter. Heating of the RTD sensor occurs byapplying a large current (e.g., about 1 mA to about 200 mA) to thesensor. The value of the applied current depends on the temperaturedesired for the RTD sensor. Generally, an RTD sensor that is used tomeasure temperature only is powered by low currents (e.g., less thanabout 0.1 mA) sufficient to obtain a value that correlates with atemperature.

Due to the relatively high current that may be used to heat an RTDsensor operating in a dual role as a temperature sensor and a heatsource, the RTD sensor may be configured in a standard four-wire mode,with four wires connected to a single, two-terminal device such that twowires are attached to each end or terminal of the RTD sensor. Two of thefour wires are then configured in a loop or circuit that applies thehigh current to the RTD sensor, via, for example, a programmable currentsource. The other two wires can be configured in a loop or circuit thatmeasures the voltage across the RTD sensor. One of the benefits of afour-wire configuration for measuring voltage across an RTD sensor isthat it minimizes errors that can be caused by lead wires to the RTDsensor.

A closed-loop controller, such as a proportional-integral-derivativecontroller (PID controller), can be desirable for controlling the RTDsensor because the device includes feedback mechanisms useful formaintaining the temperature of the RTD sensor at a constant value abovethe internal temperature of the meter. For example, a PID controller maybe used to apply a variable current to an RTD sensor that rangesanywhere from about 1 mA to about 200 mA. Furthermore, depending on thedesired temperature increase above the internal meter temperature, itcan take a period of time for a heat flux from the RTD sensor to theinside of the meter, ϕ_(INSIDE), to reach a steady state, orequilibrium, and for the temperature difference between the RTD sensorand the internal meter temperature to be maintained, based on therelationship of T_(RTD)−T_(AMBIENT) being equal to an approximatelyconstant value. In one embodiment, a temperature increase in the RTDsensor of approximately 5 degrees Celsius over the internal metertemperature is attained in less than about one minute, with the ambienttemperature being accurately determining within about ±0.5 degreesCelsius. In another non-limiting and exemplary embodiment, a temperatureincrease in the RTD sensor of approximately 2.5 degrees Celsius over theinternal meter temperature is attained in less than about 30 seconds,with the ambient temperature being accurately determining within about±1 degree Celsius.

The heat flux between the inside of the meter and the exterior ambientenvironment is in a dynamic state until the desired constant temperaturedifference is reached, at which point the heat flux is desirablymaintained in a steady state condition. The time to reach the steadystate condition can be affected by a number of factors associated withthe physical design of the fluid-analyte meter including meter casingmaterial properties (e.g., thermal conductivity) and/or structuralaspects of the meter (e.g., meter casing thickness). It is contemplatedthat it may be desirable for an instrument, such as a fluid-analytemeter, to be constructed such that the time needed for the RTD to reachsteady state, or thermal equilibrium, is minimized. For example, themeter casing or shell can be designed to reduce the thermal mass aroundthe RTD. Furthermore, the location of the RTD on or within the metercasing can be chosen to minimize the chance that the user of the metertouches or is near the RTD during meter operation.

By maintaining the temperature of an RTD sensor at an approximatelyconstant value above the internal temperature of the meter, a heat fluxfrom the RTD sensor to the inside of the meter, ϕ_(INSIDE), is expectedto remain approximately the same. Meanwhile, a heat flux going from theRTD sensor to the ambient environment, ϕ_(AMBIENT), is expected to behigher. If the total amount of heat, ϕ_(TOTAL), is known and withϕ_(INSIDE) staying constant, then the value for ϕ_(AMBIENT) can bedetermined using the relationships described in Equations 1 and 2:

ϕ_(TOTAL)=ϕ_(INSIDE)+ϕ_(AMBIENT)  (Equation 1)

ϕ_(AMBIENT)=ϕ_(TOTAL)−ϕ_(INSIDE)  (Equation 2)

The value of ϕ_(TOTAL) can be determined or approximated based on theinput energy to the RTD sensor and can be expressed by the product ofthe input voltage and current applied to the RTD sensor for bringing theRTD sensor to a steady state temperature value above the internal metertemperature. Using Equation 2, ϕ_(AMBIENT) can be determined from theknown information about ϕ_(TOTAL) and ϕ_(INSIDE) at the steady statetemperature for the RTD sensor. Equations 3 and 4 can then be applied todetermine the ambient temperature, T_(AMBIENT):

ϕ_(AMBIENT) =K _(AMBIENT)(T _(RTD) −T _(AMBIENT))  (Equation 3)

T _(AMBIENT) =T _(RTD)−(ϕ_(AMBIENT) /K _(AMBIENT))  (Equation 4)

K_(AMBIENT) is a fixed coefficient related to the surface area affectedby the heat flow. The coefficient is derived by placing an instrumentcontaining the RTD sensor (e.g., a fluid-analyte meter) into anenvironment having a known ambient temperature. The instrument is thencalibrated to determine K_(AMBIENT) experimentally. The calibrationprocess is useful because the value of K_(AMBIENT) will vary due tomechanical design changes and variations in the assembly of the meterand RTD sensor. These factors make it desirable to apply a calibrationprocess for determining the instrument-specific (e.g., meter-specific)value of K_(AMBIENT).

The embodiments in FIGS. 1 and 2 are illustrated using an RTD sensor,which is a low-resistance temperature measurement device that hasdesirable applications for precision measurements. As discussed above,an RTD sensor can be heated by applying a current across the RTD sensor,and thus, can also operate as a heat source. In certain embodiments, acontrollable heat source separate from, but near or immediately adjacentto a temperature sensor, can be used. Such a configuration allows othertypes of temperature sensors known in the art to be combined with theheat source so that a constant temperature above the internaltemperature of the instrument can be maintained while generating a heatflux across the interior and exterior of the instrument.

Referring now in more detail to FIG. 1, a meter 100 is illustratedhaving two temperature sensors 110, 150, including one RTD sensor 110for determining ambient temperature. The meter 100 includes a housing120 (e.g., protective casing or shell) defining an inside area 170 ofthe meter 100. The inside area 170 can include a printed circuit board(“PCB”) 160 having electronics associated with the meter disposedthereon. Such electronics may include a controller for the meter 100.The PCB 160 can have an internal temperature sensor 150, such as athermistor or other temperature sensing device, disposed thereon ordisposed within the electronics on the PCB 160. The PCB 160 can beconductively and/or communicatively connected to one or both of sensors110, 150. The housing 120 may be constructed of a polymer material orother suitable materials for an electronic fluid-analyte measuringinstrument (e.g., a blood glucose meter).

The housing 120 can include an interior surface exposed to the insidearea 170 and an exterior surface exposed to an ambient environment 180.The interior surface of the housing 120 can include an RTD sensor 110disposed between the housing 120 and an insulating cover 140 (e.g., acover layer) that isolates the RTD sensor 110 from the inside area 170.In certain embodiments, the RTD sensor can have the dual tasks ofoperating as a temperature sensor and as a heat source when highcurrents are applied to the sensor. The isolation of the RTD sensor 110is directed to shielding the RTD sensor 110 from temperaturefluctuations in the inside area 170 and to shield the inside area 170from temperature increases when the RTD sensor 110 operates as a heatsource. The RTD sensor 110 can also be at least partially encased orcradled within a metal layer 130 (e.g., grommet-like device) configuredto surround the RTD sensor 110 on multiple sides and further configuredin certain embodiments to be embedded with the casing. In certainembodiments, the insulating cover 140 isolates both the RTD sensor 110and the metal layer 130. As the metal layer 130 is a conductive layer,it can establish the heat flux from the RTD sensor 110, through thehousing 120, and to the ambient environment 180. The metal layer 130 canalso establish a heat flux from the ambient environment 180, through theRTD sensor 110, and into the inside area 170.

It is contemplated that in certain situations, the internal temperaturesensor 150 can provide an accurate assessment of ambient temperature.For example, after the meter has been turned on following a significantperiod of little energy consumption by the meter electronics, negligibleinternal heating of the meter 100 would be expected, and sensor 150 canbe expected to be at or near an ambient temperature. However, aftercertain heat-generating operations, the internal temperature sensor 150alone may not be able to provide an accurate estimation of ambienttemperature. As described above, the configuration described for RTDsensor 110 can be desirable because it allows for an indirectdetermination of the ambient temperature in ambient environment 180.Furthermore, it allows localized heating of the housing 120, whichfurther encourages air flow or circulation of air along the exterior ofthe housing 120, or a more accurate exposure to the ambient environment180.

An RTD sensor normally exhibits low resistance characteristics. However,when current is applied to the RTD sensor 110 in FIG. 1, the RTD sensor110 generates heat and the temperature of the RTD sensor 110 increases,particularly as the period of time of current application increases.Furthermore, as the temperature of the RTD sensor 110 rises, so does theresistance, which can lead to greater temperature increases. Forexample, in certain embodiments, an RTD sensor, such as a 100 ohmplatinum RTD sensor, may be calibrated to exhibit approximately 100 ohmsof resistance at zero degrees Celsius. The same RTD sensor may alsoexhibit approximately 120 ohms of resistance when the temperatureincreases to approximately 50 degrees Celsius. It is contemplated thatdifferent RTD sensors may be used in the embodiments described herein.For example, in one non-limiting embodiment it may be desirable to use acommercially-available Pt-385 RTD sensor. Another non-limiting exampleof an RTD sensor is the OMEGAFILM® flat profile thin film platinumsensor with ceramic base and glass coating, Model No. F2020-100-B-100,as manufacturer by Omega Engineering, Inc. of Stamford, Conn., USA.Again, different RTD sensors can be used, or different types oftemperature sensors in combination with a nearby heat source.

Referring now to FIG. 2, another embodiment is illustrated of a meter200 having two temperature sensors 210, 250, including an RTD sensor 210for determining ambient temperature. The meter 200 includes a housing220 (e.g., protective casing or shell) defining an inside area 270 ofthe meter 200. The inside area 270 can include an internal temperaturesensor 250, such as a thermistor or other temperature sensing device,disposed within the inside area 270. The housing 220 can be constructedof a polymer material or other suitable materials for an electronicfluid-analyte measuring device. The housing 220 can include an interiorsurface exposed to the inside area 270 and an exterior surface exposedto an ambient environment 280. The housing 220 can include an RTD sensor210 embedded between the interior and exterior surface of the housing220. The housing material may have insulative properties that isolatethe RTD sensor 210 from the inside area 270. In certain embodiments, theRTD sensor 210 can have the dual tasks of operating as a temperaturesensor with the application of low, constant currents and as a heatsource with the application of high variable currents for maintaining aconstant temperature above the internal meter temperature. The RTDsensor 210 can be at least partially encased or covered by a metal layer230 (e.g., metal cover) configured to surround the upper surfaces of RTDsensor 210 and further configured for certain embodiments to be exposedto the ambient environment 280. The metal layer 230 may be flush orrecessed below the exterior surface of the housing 220. It is alsocontemplated that the metal layer 230 may extend out from the exteriorsurface of the housing 220. As the metal layer 230 is a conductivelayer, it can be used to establish the heat flux from the RTD sensor 210to the ambient environment 280. The metal layer 230 can also establish aheat flux from the ambient environment 280, through the RTD sensor 220,and into the inside area 270.

In the embodiments discussed above in FIGS. 1 and 2, values obtainedfrom certain features of meters 100, 200 can be used in Equations 1through 4 to calculate the temperature of ambient environments 180, 280.For example, T_(RTD) represents the ambient temperature determined usingthe temperature information obtained from RTD sensor 110 or RTD sensor210. T_(INSIDE) represents the internal or inside temperature of themeters 100, 200 determined using the temperature information obtainedfrom the internal temperature sensor 150 or the internal temperaturesensor 250. The temperature, T_(RTD), of the RTD sensors 110, 210 iscontrolled by applying a variable current to the sensor via a controlleror otherwise so that the sensor reaches and maintains a steady-statetemperature value of T_(INSIDE) plus a fixed constant value. The fixedconstant may vary depending on the application. In certain embodiments,the fixed constant value may range from 1 to 10 degrees Celsius. Thatis, the temperature of the RTD sensor 110, 210, or T_(RTD), is increasedor heated by a fixed amount that can range from approximately 1 to 10degrees Celsius above the temperature, T_(INSIDE), determined by theinternal temperature sensors 150, 250. In certain embodiments, the fixedconstant may be greater than or less than the 1 to 10 degree Celsiusrange. In certain embodiments, the fixed constant of T_(RTD) at itssteady state value minus T_(INSIDE) is equal or slightly greater thanthe maximum internal temperature rise of the meter 100, 200. It is alsocontemplated that in certain embodiments, the temperature of RTD sensors110, 210 may be maintained at a temperature that is lower than theinternal temperature of the meter by using thermoelectric cooling, suchas, through the Peltier effect. By applying thermoelectric cooling, thetemperature of the RTD sensor may be decreased by up to several degreesand maintained a constant temperature that is lower than the internalmeter temperature. As discussed herein, the internal temperature of themeter may be affected by many elements, including internal electroniccomponents in the meter or nearby heat sources that cause heat to flowthrough the meter housing and into the inside areas of the meter.

As discussed above in Equations 1 and 2, total heat flux is equal to thesum of ϕ_(INSIDE) (i.e., the heat flow from the RTD sensor to the insidearea of the meter) and ϕ_(AMBIENT) (i.e., the heat flow from the RTDsensor to the ambient environment). The heat flux from the RTD sensor tothe inside area of the meter and the heat flux from the RTD sensor tothe ambient environment can be represented by the relationshipsdescribed in Equations 5 and 6:

ϕ_(INSIDE) =k _(INSIDE) A _(INSIDE)(T _(RTD) −T _(INSIDE))  (Equation 5)

ϕ_(AMBIENT) =k _(AMBIENT) A _(AMBIENT)(T _(RTD) −T_(AMBIENT))  (Equation 6)

where A_(INSIDE) is the internal surface area over which the heat flows,A_(AMBIENT) is the external surface area over which the heat flows, andk_(INSIDE) and k_(AMBIENT) are fixed coefficients. The internal surfacearea, A_(INSIDE), in the FIG. 1 and FIG. 2 embodiments can be estimatedto be the surface area of metal layer 130, 230 facing the respectiveinside areas 170, 270 of the meter 100, 200. The external surface area,A_(AMBIENT), can be estimated to be the surface area of metal layer 130,230 facing the respective ambient environments 180, 280 of the meter100, 200.

In one non-limiting embodiment of a temperature sensor configuration fora fluid-analyte meter, a 2×2×0.8 mm thin-film RTD sensor (e.g., OmegaModel No. F2020-100-B-100) is embedded into the outer shell or casing ofthe meter and has a resistance of 100 ohms at zero degrees Celsius. Ametal layer or heat radiator (e.g., 130, 230) is fabricated from aheat-conducting material, such as copper, and is fitted into theexterior shell over the RTD sensor such that part of the metal layer isalong the exterior surface of the shell for the meter. An internaltemperature sensor (e.g., 150, 250) such as a chip thermistor is locatedwithin the inside area of the meter and has a value of 50 kOhm at 25degrees Celsius. The chip thermistor can be mounted, for example, to aPCB within the meter. The outer casing or shell of the meter may befabricated of a plastic material, and one non-limiting embodiment hasdimensions of approximately 110×60×20 mm. The internal temperature ofthe meter may rise due to various heat sources associated with themeter, including various electrical components prone to heat rise. Thetypical heat generated within a meter or other type of handheldelectronic instrument can come from various sources and can beapproximated by the application of 100 mW to 5 W of total power tovarious resistive components within the handheld device during itsoperation. This heat can raise the internal temperature of the exemplarymeter by approximately 5 to 30 degrees Celsius above the ambienttemperature. Furthermore, the internal temperature may be fluctuating asthe operations of the meter change. As described earlier, a PIDcontroller may be used to supply current to the RTD sensor to maintainthe RTD sensor at a fixed temperature value (e.g., about 5 degreesCelsius, about 10 degrees Celsius) above the internal temperature, asmeasured by the chip thermistor. The current supplied to the RTD sensorcan range from about 1 mA to about 30 mA. The total power applied to theRTD sensor can also be monitored and limited to about 0.1 mW to about 90mW. By applying Equations 1 to 6 for pre-determined values of K, theambient temperature can then be determined for the meter.

The time needed to determine the ambient temperature via the exemplaryembodiments described herein can vary. In certain embodiments, the timeto determine ambient temperature can range up to about 30 seconds. Incertain embodiment, the time is less than about 20 seconds. In otherembodiments, the time may exceed 30 seconds. In further embodiments, theambient temperature can be determined in less than about 60 seconds. Itcan be particularly desirable to determine ambient temperature withinthe time ranges described herein for the described exemplary embodimentsbecause such time ranges allow a user to obtain timely ambienttemperature data appropriate for accurately determining, for example,temperature-sensitive fluid-analyte concentrations. The embodimentsdescribed herein are also useful for any temperature-sensitiveapplication that requires accurate and up-to-date ambient temperaturedata. In the above non-limiting embodiment, the ambient temperature isdetermined to within about ±1 degree Celsius of the actual ambienttemperature. Again, the above values apply to but one of numerousembodiments contemplated to fall within the present disclosure.

While the product of k_(INSIDE) and A_(INSIDE) can be approximated asbeing constant, the product may actually fluctuate over a widetemperature range. For example, in the above non-limiting embodiment, asthe internal temperature fluctuates from 5 to 45 degrees Celsius, theproduct of k_(INSIDE) and A_(INSIDE) may range from about 0.56 at 5degrees Celsius to about 0.63 at 45 degrees Celsius. Compensation forthis fluctuation can be done for meters expected to be used over a wideoperating temperature range by applying a linear fit or other best-fitequation to approximate the value for the product of k_(INSIDE) andA_(INSIDE).

As discussed above for Equations 3 and 4, once ϕ_(AMBIENT) is determinedfrom known information about ϕ_(TOTAL) and ϕ_(INSIDE) at the steadystate temperature, T_(AMBIENT) can be determined from the product ofk_(AMBIENT) and A_(AMBIENT), or K_(AMBIENT), which is a derived constantthat is specific to a given meter configuration. As discussedpreviously, A_(INSIDE) is the internal surface area over which the heatflows, A_(AMBIENT) is the external surface area over which the heatflows, and k_(INSIDE) and k_(AMBIENT) are fixed coefficients. Theproduct of k_(INSIDE) and A_(INSIDE) (K_(INSIDE)) and the product ofk_(AMBIENT) and A_(AMBIENT) (K_(AMBIENT)) are derived values for aspecific meter configuration under certain operating conditions.

As discussed previously, a calibration process is performed to derivethe values for K_(INSIDE) and K_(AMBIENT). In one non-limiting exemplarycalibration process, the calibration is performed at a constant ambienttemperature (e.g., about 25 degrees Celsius) with the RTD sensoroperating at a fixed temperature value above the ambient temperature(e.g., about 25+10 degrees Celsius). As an initial step, any heatsources associated with the meter are turned off or removed, and the RTDsensor is maintained at 10 degrees Celsius above the internaltemperature. Theoretically, the internal temperature of the meter willbe higher than the ambient temperature due to, for example, the heatgenerated by the RTD sensor. However, the heat generated by the RTDsensor may be very small (e.g., between 10 to 20 mW), and thus, theremay be no noticeable difference between the internal temperature of themeter and the ambient temperature when all other heat sources associatedwith the meter are isolated. In the next step, a heat source isactivated within the meter, which raises the internal temperature of themeter. The temperature of the RTD sensor is maintained at a constantvalue above the internal temperature of the meter as determined by thechip thermistor (e.g., internal meter temperature +10 degrees Celsius).The power supplied to the RTD sensor is also monitored along with theinternal temperature of the meter. The constants K_(INSIDE) andK_(AMBIENT) can then be calculated using Equations 7 and 8, which arebased on maintaining the non-limiting and exemplary fixed constant of 10degrees Celsius above the internal temperature:

K _(INSIDE)=(T _(INTERNAL HEAT) −T _(AMBIENT)−10)/(P _(HEAT RTD) −P_(INITIAL RTD))  (Equation 7)

K _(AMBIENT)=(P _(INITIAL RTD)−10*K _(INSIDE))/10  (Equation 8)

-   -   where T_(INTERNAL HEAT) is the internal temperature after        applying the heat source;        -   T_(AMBIENT) is the known ambient temperature;        -   P_(HEAT RTD) is power supplied to RTD sensor through            T_(INTERNAL HEAT); and        -   P_(INITIAL RTD) is power supplied to RTD sensor before            internal heating.            Other fixed constants are contemplated in addition to +10            degrees Celsius (e.g., +3 degrees, +5 degrees, +7 degrees,            +15 degrees), which is only used for purposes of            illustrating one demonstrative aspect of the present            disclosure. Furthermore, other applied currents may be used            to calibrate a meter depending on the desired operating            conditions.

The temperature sensor embodiments disclosed herein provide aneconomical way to determine ambient temperature surrounding a meter thatin turn provide multiple useful applications such as increased accuracyin determining sample temperature, minimizing human error whileoptimizing meter operations by allowing an auto-start capability in themeter once a desirable temperature range is reached, determining thereadiness of the sample for testing based on the reagent falling withina desirable temperature range, and/or reducing temperature-relatederrors during analyte concentration determinations of a fluid sample.

Exemplary and non-limiting systems that may employ the above describedtemperature sensor embodiments are described further below in theexample of FIGS. 3-5.

Turning now more generally to FIG. 3, a fluid analyte system 300 isillustrated including a meter 310 with a port for receiving andanalyzing a fluid sample on a test sensor 320. The test sensor includesa connection end 330 where the test sensor 320 interfaces with the meter310. The interface between the meter 310 and the test sensor 320 canallow the meter 310 to energize the test sensor 320 by applying, forexample, a voltage difference across contacts on the test sensor and themeter. The test sensor 320 also includes a fluid-receiving end 340 forreceiving a fluid sample into a fluid-receiving area 343 for subsequentanalysis using the meter 310. A first temperature sensing element 316can be positioned in close proximity to the fluid-receiving area 343 ofthe test sensor 320. In addition to, or alternatively, a secondtemperature sensing element 336 may be positioned within or at thesurface of the shell or casing of the system 300.

Analytes that may be determined using the device include glucose, lipidprofiles (for example, cholesterol, triglycerides, LDL and HDL),microalbumin, hemoglobin A1c, fructose, lactate, or bilirubin. Thepresent invention is not limited, however, to devices for determiningthese specific analytes, and it is contemplated that other analyteconcentrations may be determined. The analytes may be in, for example, awhole blood sample, a blood serum sample, a blood plasma sample, orother body fluids like ISF (interstitial fluid) and urine.

In FIG. 3, the meter 310 receives and engages the test sensor 320. Themeter 310 measures the concentration of analyte for the sample collectedby the test sensor 320. The meter 310 can include contacts for theelectrodes to detect the electrochemical reaction of an electrochemicaltest sensor. Alternatively, the meter 310 can include an opticaldetector to detect the degree of light alteration for an optical testsensor. To calculate the actual concentration of analyte from theelectrochemical reaction measured by the meter 310 and to generallycontrol the procedure for testing the sample, the meter 310 employs atleast one processor 312, which may execute programmed instructionsaccording to a measurement algorithm. Data processed by the processor312 can be stored in a memory 314. The meter 310 may also use the sameor a different processor for various operations, such as, for example,power management or temperature functions, including executing routinesfor temperature prediction of ambient temperature. Furthermore, themeter can include a user interface.

The temperature sensing elements 316, 336 can include, among otherthings, resistance temperature devices (see, e.g., FIGS. 1 and 2),thermistors, diode devices, etc. For non-RTD sensors, it is contemplatedthat temperature sensing elements 316, 336 will also include a nearbyheat source (not shown) so that the heat flux through the system shellor casing can be determined. In addition, a third temperature sensingelement 346 can be disposed within the interior of meter 310 to assessthe internal meter temperature. The third temperature sensing elementmay be disposed on or near a PCB. In certain embodiments, the internaltemperature sensor can also be embedded in a microcontroller 318 that isdisposed within the meter 310. The third temperature sensor is connectedto a processor or a microcontroller of the meter to allow absolutetemperature readings to be collected within the meter itself. The meter310 may also use the same or a different microcontroller or processorfor power management, temperature prediction operations, data transferoperation, or to execute other routines associated with the meter 310.

FIG. 4 illustrates an exemplary embodiment of a fluid analyte system. Inparticular, a portable meter 400 includes some or all of the elementsdiscussed for the embodiments described in FIG. 3 and elsewhere herein.As shown in FIG. 4, the meter 400 includes a display 402 visible througha front portion 420, a test-sensor port 404, and a plurality of buttons406 a, 406 b. After a user places a sample fluid on a test-sensor thatis inserted into the test sensor port 404, the fluid analyte (e.g.,glucose) level is determined by the meter 400, which displays the fluidanalyte (e.g., glucose) reading on the display 402. The reading is thenstored in the meter's memory device.

The meter 400 includes a microprocessor or the like for processingand/or storing data generated during the testing procedure. The meter400 may also use the same or a different microprocessor for powermanagement or temperature operations, including executing routines tocontrol recharging operations of the meter 400 for battery-operateddevices and for implementing temperature prediction algorithms inassessing ambient temperatures.

The test sensor port 404 is adapted to receive and/or hold a test sensorand assist in determining the analyte concentration of a fluid sample. Ameter temperature may be monitored for the meter 400 with an internalmeter temperature sensor 460 located within an inside area of the metershell or casing. Another temperature sensor 462, such as an RTD sensor,may be embedded within or along a surface of the meter casing.

It is contemplated that in certain embodiments, the fluid analytesystems illustrated in FIGS. 3 and 4 can be configured to include alancing device (not shown). For example, meters 300, 400 can beconfigured to receive and engage an element that collects a fluidsample. The meter 300, 400 can measure the concentration of analyte forthe sample collected by the lancing device. The meter 300, 400 can alsoinclude contacts connected to electrodes that detect electrochemicalreactions of an electrochemical test sensor within the lancing device.In certain embodiments, the fluid analyte system may be an integratedsystem that receives samples, processes analyte concentrations of fluidsample, and/or stores data within a self-contained system including themeter and lancing device.

FIGS. 5A and 5B illustrate an exemplary fluid analyte meter embodiment.Fluid analyte meter 500 can include some or all of the functionalitiesand components discussed for the embodiments described in FIGS. 3 and 4.For example, the fluid analyte meter 500 can be a portable blood glucosemeter that is an integrated device with certain data processing anddisplay features. A user can employ the fluid analyte meter 500 toanalyze a blood sample by inserting a test sensor into port 520. A portlight, such as, a port light emitting diode 525 may be disposed near theport 520 to illuminate the port area and assist the user with insertingthe test sensor. The fluid analyte meter 500 can also include a battery580 that may be recharged by a connection via a USB interface element570 to either an external processing device (not shown), such as a PC,or other external power supply. If a rechargeable battery is used, acharging integrated circuit 545 may be included in meter 500 forrecharging the battery 580. In certain embodiments, a battery may bedisposed in a cap 502, which fits over the USB interface element 570.The meter 500 can also include a display 550 that provides informationto a user of the meter 500. For example, the display 550 can includeinformation on the battery strength, a calculated analyte concentration,historical analyte concentrations, date and time data, and power on/offinformation.

The fluid analyte meter 500 can also include one or more thermistors orother types of temperature sensing devices. For example, an RTD sensor530 can be disposed within or at a surface of the outer casing or shellof the meter 500. A microcontroller with an embedded temperature sensor540 can also be disposed within the meter 300 to determine the internalmeter temperature. The RTD sensor 530 and/or temperature sensor 540 areconnected to a processor or a microcontroller of the meter 500 to allowtemperature readings to be collected. The meter 500 may also use thesame or a different microcontroller or processor for power management,temperature prediction operations, data transfer operation, or toexecute other routines associated with the meter 500. For example,temperature prediction algorithms can be implemented on themicrocontroller or processor to determine an accurate ambienttemperature for use in calculating an analyte concentration.

In certain embodiments, a fluid-analyte meter is configured to rapidlydetermine ambient temperature. The meter may include a housing definingan interior space and an exterior ambient space. The meter may alsoinclude a processor, a controller, a first and second temperaturesensor, and a temperature-adjustment source. The processor may bedisposed within the housing. The controller may also be disposed withinthe housing and be communicatively connected to the processor. The firsttemperature sensor may be disposed within the interior space of thehousing. The second temperature sensor may be disposed on the housingand be communicatively connected to the controller. Thetemperature-adjustment source may be disposed on the housing andconfigured to create a convective zone about the second temperaturesensor. The controller is configured to transmit instructions forsupplying a variable current to the temperature-adjustment source suchthat the temperature-adjustment source adjusts the second temperaturesensor to a predetermined steady-state temperature value different froma first temperature value. The first temperature value is based on firsttemperature data received via the controller from the first temperaturesensor. The controller is further configured to receive secondtemperature data from the second temperature sensor. The controller orthe processor is configured to determine an ambient temperature of theexterior ambient space based on the supplied current values, thepredetermined steady-state temperature, and the first temperaturevalues.

In certain embodiments, the fluid-analyte meter described above may alsobe configured with one or more of the following features. Thefluid-analyte meter can be configured with the second temperature sensorand the temperature-adjustment source being a single element. The secondtemperature sensor can also be an RTD sensor. The temperature-adjustmentsource may also be a heat source and the pre-determined steady-statetemperature value may be above the first temperature value.Alternatively, the temperature-adjustment source may be a thermoelectriccooler and the pre-determined steady-state temperature value may bebelow the first temperature value. The ambient temperature of theexterior space may be determined in less than one minute, or the ambienttemperature of the exterior space may be determined in less than twentyseconds. It is also contemplated that the second temperature sensor isembedded in an interior surface of the housing. The second temperaturesensor can also be embedded in an exterior surface of the housing. Thesecond temperature sensor may also be at least partially embedded in ametal layer, where the metal layer is at least partially embedded in thehousing. The meter can also include a printed circuit board disposed inthe interior space of the housing with the first temperature sensorbeing disposed on the printed circuit board.

In certain embodiments, a method for rapidly determining ambienttemperature in a fluid-analyte meter includes providing a meterincluding a housing with a first temperature sensor disposed within aninterior space of the housing. A second temperature sensor is disposedon the housing, and a temperature-adjustment source is disposed near thesecond temperature sensor. A first temperature value is determined fromfirst temperature data received via the first temperature sensor. Avariable current is applied to the temperature-adjustment source suchthat the second temperature sensor is adjusted to a predeterminedequilibrium temperature value different from the first temperaturevalue. A second temperature value is determined from second temperaturedata received from the second temperature sensor. An ambient temperatureof an exterior space of the fluid-analyte meter is determined based onthe variable current and the determined first and second temperaturevalues.

In certain embodiments, the method for rapidly determining ambienttemperature can also include one or more of the following features. Thesecond temperature sensor and the temperature-adjustment source can be asingle element. The second temperature sensor can also be an RTD sensor.The temperature-adjustment source can be a heat source and thepre-determined equilibrium temperature value can be above the firsttemperature value. Alternatively, the temperature-adjustment source canbe a thermoelectric cooler and the pre-determined equilibriumtemperature value can be below the first temperature value. The secondtemperature sensor can be embedded in an interior surface or an exteriorsurface of the housing. The second temperature sensor can also be atleast partially embedded in a metal layer, where the metal layer is atleast partially embedded in the housing. The method can also includeproviding a printed circuit board disposed in the interior space of thehousing, where the first temperature sensor is disposed on the printedcircuit board.

In certain embodiments, a portable meter is configured to rapidlydetermine ambient temperature. The portable meter includes a housing, aprocessor, and a first and second temperature sensor. The housingdefines an interior space and an area for receiving a fluid sample. Theprocessor and the first temperature sensor are disposed within theinterior space of the housing. The second temperature sensor is disposedon the housing. The processor is configured to determine a firsttemperature value from temperature data received from the firsttemperature sensor. The processor is further configured to apply avariable current to a temperature-adjustment source such that the secondtemperature sensor is adjusted to a predetermined steady-statetemperature value different from the first temperature value. Theprocessor is also configured to determine an ambient temperature of anexterior space of the housing based on the applied variable current, thepre-determined steady-state temperature value, and the received firsttemperature value.

In certain embodiments, the above portable meter is configured toinclude one or more of the following features. The second temperaturesensor and the temperature-adjustment source can be a single elementcomprising an RTD sensor. The second temperature sensor can be embeddedin an interior surface of the housing. The second temperature sensor canbe embedded in an exterior surface of the housing. The secondtemperature sensor can be at least partially embedded in a heatconductive layer, with the heat conductive layer is at least partiallyembedded in the housing. The portable meter can also include a printedcircuit board disposed in the interior space of the housing, with thefirst temperature sensor and the processor being disposed on the printedcircuit board. The temperature-adjustment source can be a heat sourceand the pre-determined steady-state temperature value can be above thefirst temperature value. Alternatively, the temperature-adjustmentsource is a thermoelectric cooler and the pre-determined steady-statetemperature value can be below the first temperature value. A heat fluxfrom the second temperature sensor to the interior space of the housingmay be approximately constant upon attaining the predeterminedsteady-state temperature value at the second temperature sensor. A totalapplied energy can also be determined based on the applied variablecurrent. It is also contemplated that the ambient temperature of theexterior space can be determined in less than one minute, or in lessthan twenty minutes.

It would be understood within the field of the present disclosures thatelements and/or components of the meter modules and/or portable devicesdescribed herein can be embodied in a single device or in multipledevices in various configurations of elements and/or components.Furthermore, it would be understood that the devices described hereincan be used in portable or non-portable fluid analyte meters. Thus,while the meter modules or portable devices described herein may beportable, the present disclosures can also be applied to non-portablefluid analyte meters.

While the invention has been described with reference to details of theillustrated embodiments, these details are not intended to limit thescope of the invention as defined in the appended claims. For example,although the illustrated embodiments are generally described for RTDsensors on the casing or shell of a meter, other temperature sensors maybe used that are associated with a heat source. Furthermore, differenttypes of temperature sensors may be used to determine the temperature ofthe inside area of the meter. In addition, the calibration process maybe modified for a specific meter application generally using theprocedures described herein. In addition, it should be noted that thecross-section of the meters and test sensors used herein may be othershapes such as circular, square, hexagonal, octagonal, other polygonalshapes, or oval. The non-electrical components of the illustratedembodiments are often made of a polymeric material. Non-limitingexamples of polymeric materials that may be used in forming the meterinclude polycarbonate, ABS, nylon, polypropylene, or combinationsthereof. It is contemplated that the fluid analyte systems can also bemade using non-polymeric materials. The disclosed embodiments andobvious variations thereof are contemplated as falling within the spiritand scope of the claimed invention.

1-26. (canceled)
 27. A portable instrument for determining a bloodglucose concentration of a blood sample, the portable instrumentincluding systems for rapidly determining an ambient temperature, theportable instrument comprising: a protective casing defining an interiorspace, an area for receiving a blood sample, and an exterior ambientenvironment; a physical processor and a first temperature sensordisposed within the interior space; and a second temperature sensordisposed at least partially within the protective casing; and one ormore memory devices encoded with instructions, that upon execution bythe physical processor or another physical processor, cause the portableinstrument to implement the following acts: determine a firsttemperature value from temperature data received from the firsttemperature sensor, apply a variable current to a temperature-adjustmentsource such that the second temperature sensor is adjusted to apredetermined steady-state temperature value different from the firsttemperature value, and determine an ambient temperature of the exteriorambient environment, the ambient temperature based on the appliedvariable current, the pre-determined steady-state temperature value, andthe received first temperature value.
 28. The portable instrument ofclaim 27, wherein the variable current is in a range between 1 mA and200 mA.
 29. The portable instrument of claim 27, wherein the secondtemperature sensor is adjusted to a predetermined steady-statetemperature value up to about 10 degrees Celsius greater than the firsttemperature value.
 30. The portable instrument of claim 27, wherein theambient temperature is determined to within about ±1 degree Celsius ofan actual ambient temperature.
 31. The portable instrument of claim 27,wherein the second temperature sensor and the temperature-adjustmentsource are a single element.
 32. The portable instrument of claim 27,wherein the second temperature sensor is an RTD sensor.
 33. The portableinstrument of claim 27, wherein the second temperature sensor isembedded in an interior surface of the protective casing.
 34. Theportable instrument of claim 27, wherein the second temperature sensoris embedded in an exterior surface of the protective casing.
 35. Theportable instrument of claim 27, wherein the second temperature sensoris at least partially embedded in a heat conductive layer, the heatconductive layer being at least partially embedded in the protectivecasing.
 36. The portable instrument of claim 27, further comprising aprinted circuit board disposed in the interior space of the protectivecasing, the first temperature sensor and the physical processor beingdisposed on the printed circuit board.
 37. The portable instrument ofclaim 27, wherein a heat flux from the second temperature sensor to theinterior space of the protective casing is approximately constant uponattaining the predetermined steady-state temperature value at the secondtemperature sensor.
 38. The portable instrument of claim 27, wherein theone or more memory devices are encoded with instructions, that uponexecution by the physical processor or the another physical processor,cause the portable instrument to further implement that act ofdetermining a total applied energy based on the applied variablecurrent.
 39. A method for rapidly determining ambient temperature in aportable blood-glucose meter, the method comprising: providing aportable blood glucose meter including: (i) a protective casing definingan interior space, an area for receiving a blood sample, and an exteriorambient environment, (ii) a first temperature sensor disposed within theinterior space, (iii) a second temperature sensor disposed at leastpartially within the protective casing, and (iv) atemperature-adjustment source disposed near the second temperaturesensor; determining a first temperature value from temperature datareceived from the first temperature sensor; applying a variable currentto the temperature-adjustment source such that the second temperaturesensor is adjusted to a predetermined steady-state temperature valuedifferent from the first temperature value; and determining an ambienttemperature of the exterior ambient environment, the ambient temperaturebased on the applied variable current, the predetermined steady-statetemperature value, and the received first temperature value.
 40. Themethod of claim 39, wherein the variable current is in a range between 1mA and 200 mA.
 41. The method of claim 39, wherein the secondtemperature sensor is adjusted to a predetermined steady-statetemperature value up to about 10 degrees Celsius greater than the firsttemperature value.
 42. The method of claim 39, wherein the ambienttemperature is determined to within about ±1 degree Celsius of an actualambient temperature.
 43. The method of claim 39, further comprisingproviding a printed circuit board disposed in the interior space, thefirst temperature sensor being disposed on the printed circuit board.44. The method of claim 39, wherein the second temperature sensor isembedded in an interior surface of the protective casing.
 45. The methodof claim 39, wherein the second temperature sensor is embedded in anexterior surface of the protective casing.
 46. The method of claim 39,wherein upon attaining the predetermined steady-state temperature valueat the second temperature sensor, a heat flux from the secondtemperature sensor to the interior space defined by the protectivecasing is approximately constant.
 47. The method of claim 39, furthercomprising determining a total applied energy based on the appliedvariable current.